The present invention relates to the production of amorphous silicon nitride or silicon carbide, and is particularly concerned with a process for producing such products, particularly amorphous or crystalline silicon nitride, having a desired morphology, for example in the form of needles.
Silicon nitride is a highly desirable refractory material having wide industrial use such as the manufacture of crucibles, cutting tools and also as a filler for plastic, rubber and silicon compounds and for mold lubrication, insulation and coatings, and including uses in adiabatic diesels, high temperature turbine blades, and the like.
For large commercial application, it is desirable to provide silicon nitride in finely divided, pure, state. Commercially available silicon nitride has relatively large undesirable particle size and when it is attempted to reduce its particle size as by comminution, the resulting product often contains undesirable impurities.
Conventionally, silicon nitride has been prepared by the nitridation of elemental silicon. However, such process often leads to production of undesirably coarse product. Further, since fine silicon powder tends to adsorb atmospheric impurities, the resulting product often is highly contaminated. Moreover, the production of fine, active silicon nitride by silicon nitridation is difficult since high purity silicon forms protective nitride layers.
It has also been proposed to prepare silicon nitride in a highly pure, controlled particle form by the reaction of silicon tetrachloride with liquid ammonia, but retention of chlorine in the product amorphous silicon nitride was a problem, which required long-term extraction with liquid ammonia to completely remove the chlorine.
U.S. Pat. No. 3,211,527 to Forsyth discloses production of fine silicon nitride by reaction of silicon monosulfide vapor and ammonia at temperatures ranging from about 1250.degree. C. to 1500.degree. C. in a reactor, and cooling and collecting the silicon nitride removed from the reactor.
NATO Advanced Study Institutes, Nitrogen Ceramics, Nordhoff 1977, and Progress in Nitrogen Ceramics, Nijhoff 1983, both edited by F. L. Riley, disclose the prior art for production of silicon nitride.
Silicon carbide whisker products have been formed by high temperature reaction of sand and coke. In U.S. Pat. No. 3,709,981, silicon carbide in the form of whiskers or fibers is prepared by reacting at elevated temperature of the order of 1500.degree. C., silica, elemental carbon, a source of sulfur and a source of hydrogen, such as hydrogen sulfide, and a gaseous source of carbon such as a hydrocarbon or other carbon containing compounds, e.g. carbon disulfide.